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Thursday, 25 June 2015

The online palaeontological community has no shortage of words on the recently released Jurassic World movie – most of them concerning the deplorable disregard for the last two decades of dinosaur science. What of the movie itself? The critical response seems to divided, most reviewing it as a great popcorn movie, and the rest as a predictable, sexist and cynical summer film. My own take is the latter: Jurassic World was just another forgettable, contrived entry in the Jurassic Park franchise, best noted for having the worst effects, silliest plot, and most outmoded characters of the entire series. Never quite sure if it’s making fun of modern franchise culture or revelling in it, the convoluted story revolves around (SPOILER ALERT) hokey family values, dinosaur-dinosaur team ups, dinosaur-human team ups and weaponised artificial species to form a plot akin to a particularly dumb, low-grade B-movie. There are some good ideas in there that could, in isolation, make for interesting science fiction, but there’s so much going on that nothing has a chance to develop: plot threads are introduced, contradicted and abandoned with rapidity. Most of the story is churned along by really contrived, forehead-slappingly stupid decisions made by the characters, and the pandering to the ‘awesomebro’ crowd is, at times, shameless. Was I the only person cringing when two characters told us, in weirdly meta-fashion, how ‘awesome’ and ‘badass’ they thought the (already shark-jumping) motorcycle/Velociraptor scenario was? In all, while I can’t say I strongly disliked Jurassic World, its few redeeming features are undermined by the contrivances, tropes and fan-servicing, over-stuffed plot, and flat characters. After two other disappointing sequels, Jurassic World is yet another demonstration that the Jurassic franchise really needs to evolve away from the original film to remain interesting. Predictably, it’s made a truckload of money already.

Anyway, this isn’t a review of Jurassic World: we’re here to talk about dromaeosaurs (yeah, not ‘raptors’: sorry, Jurassic fans, but another set of dinosaurs have held priority to ‘raptor’ since 1873). The velociraptors are back in force in Jurassic World, in all their leathery-skinned, broken-wristed, overtoothed glory. Of all the Jurassic World dinosaurs, the velociraptors have moved furthest from being relatively ‘believable’ animals in the first movie to the realm of true sci-fi monster. By Jurassic World, the behavioural and physical attributes they’ve gained in each sequel has finally made them totally unstoppable killing machines, demonstrably invulnerable to all damage except when the script calls for it (and thus largely removing their potential for being thrilling characters or antagonists. Oh, wait, we're not reviewing the film!). Inspired by their movie cousins, I thought I’d share some recently completed dromaeosaurid palaeoart here. Without appreciating it, presenting these three images together acts as a foil to the Jurassic depiction of dromaeosaurs, showing these animals as exploitative and flawed creatures, and as products of natural evolution, rather than reptilian versions of Geiger’s Alien. As usual, prints of all these images are available to purchase from my print store.

Velociraptor: picking on the little guy

First up is Velociraptor mongoliensis, the dog-sized namesake of the Jurassic dromaeosaurs. In this revised image (the first version of which topped another Jurassic World inspired piece) Velociraptor is shown predating a much smaller theropod, a juvenile oviraptorosaur Citipati osmolskae. The idea emphasised here is that, like most predators, Velociraptor probably hunted easily dispatched and overpowered prey, like juvenile animals, rather than larger, more dangerous individuals. A distressed oviraptorosaur parent is shown in the background as attempting to scare the predator off, arms extended, jaws agape, probably making a lot of noise. It strikes me this is the ‘classic’ palaeoart pose so often depicted as leaping from canvases to our faces – I think it works a lot better in the context of a full scene rather than in isolation. The Velociraptor is adorned with two small feather fans on its snout, structures for which we have no direct evidence, but which don’t seem too audacious in light of some cranial display features of modern predators.

Deinonychus: superklutz

Deinonychus antirrhopus: Deadly. Savage. Clumsy.

Next is another famous dromaeosaur, the North American species Deinonychus antirrhopus. This image was commissioned by ReBecca Hunt-Foster for the Utah Bureau of Land Management, as part of a public display on the Mill Canyon Dinosaur Tracksite. This Cedar Mountain Formation locality, once a scummy, slimy shallow body of water, preserves a multitude of sauropod, ornithopod and theropod tracks, including several belonging to dromaeosaurs. We call these tracks Dromaeosauripus, and at Mill Canyon their most likely trackmaker is Deinoynchus, it being a Cedar Mountain Formation species of correct stratigraphic provenance and appropriate size to make these specific Dromaeosauripus traces. Some of the Mill Canyon Dromaeosauripus tracks record running animals, which is pretty neat: it’s hard not to wonder what impetus made these animals charge over the Mill Canyon microbial mat 100 million years ago.

Alongside some of these tracks are long gouges in the ancient mud seemingly made by two-toed animals losing their grip on the substate, wobbling about before regaining their balance – are these the tracks of noble Deinonychus almost falling over? Quite possibly, although it’s not definite that they record the same individuals as those leaving the charging Dromaeosauripus prints.

ReBecca thought it would be fun to demonstrate that some Mill Canyon dinosaurs weren’t the most sure-footed of creatures, and requested my services to do so. I was happy to do this. In any sustained bout of animal observation it becomes apparent that all species routinely trip, slip and blunder about in the way that we do, and recreating this seemed a wonderful alternative to our regular diet of epic and ‘awesome’ palaeoart. The fact this image features Deinonychus is even better: even outside of Jurassic Park, dromaeosaurs are regularly depicted as particularly ferocious, cunning predators, earning them the nickname of ‘lions of the Cretaceous’. Well, awesomebros, here's our noble, cunning Cretaceous lion picking a whole bunch of oopsie-daisies, while a couple of normal Deinonychus prey items – Tenontosaurus – look out from the far distance and laugh.

Achillobator: giant dromaeosaur, silly hat

The Late Cretaceous, Mongolian species Achillobator giganticus is not a household name, but that may well change over time. This species is large bodied (only second to Utahraptor in the dromaeosaurid size game) and robust, bearing a deep snout, stout limbs and a large set of hips. It probably wasn’t a fast runner, but all indications are that it was a powerful predator suited to wrestling and grappling, perhaps ideally suited to ambushing larger prey. In this illustration, I’ve speculated that the powerful limb girdles and appendages of Achillobator are for a specific purpose: digging out and killing burrowing reptiles. Lots of tetrapods, including lineages around in the Mesozoic, were burrowers: fossorial activities are known in extinct and modern dinosaurs, as well as crocodylomorphs, certain lepidosaurs, and stem-mammals. The Mesozoic was thus likely full of burrowing species, and it’s not crazy to think that powerful predators could trap and excavate these animals from their own homes with the right equipment. In the scenario depicted above, the robust feet and enlarged hips of Achillobator make for powerful digging tools, while the short but powerful arms are ready to catch and grapple with escaping animals. If their prey doesn’t emerge voluntarily, those feather crowns will block burrow entrances once the head and jaws are inserted to extract the animals directly – this is a nod to the cranial form of Tibetan foxes, which have wide heads for the same purpose.

Of course, this image and concept is little more than an All Yesterdays-style speculation – to be honest, we don’t really have enough of the Achillobator skeleton to know exactly what it did for a living. Nevertheless, following another run with the Jurassic movies, I find it refreshing, grounding and intriguing to think of large dromaeosaurs as real products of evolution, as creatures adapted to the environment they lived in, and the species they coexisted with. As is often the case, reality ends up being far more interesting than fiction.

Friday, 19 June 2015

Non-pterodactyloids like Dimorphodon macronyx are meant to be slow, sprawling terrestrial locomotors. So what's up with this image of one running with erect limbs? Read on... Prints of this image are available.

Readers interested in flying reptiles will know that a recent, major paradigm shift in our understanding of these animals concerns their terrestrial locomotion. In short, perceptions of pterosaurs as awkward, clumsy terrestrial creatures have been overturned to models of upright, proficient walkers and runners. This reinterpretation is founded on a substantial amount of evidence from skeletal anatomy, trackway data, as well as reconstructions of muscle kinematics, and underpinned recent proposals of terrestrial foraging in some pterosaurs (most famously of course, the azhdarchids - read this for an overview of their research history) - famously stark departures from classic views of pterosaur palaeobiology. However, it is really only pterodactyloids, that group of Jurassic-Cretaceous pterosaurs which contains the majority of pterosaur diversity, that much of this work on terrestriality really applies to. It seems all pterosaur tracks are actually pterodactyloid tracks; most 3D fossils we've used to understand limb joints and motion are pterodactyloid fossils, and most detailed discussions of step cycles, stride lengths and so on are based on pterodactyloid proportions.

Non-pterodactyloids are considered somewhat differently. Our views on early pterosaur walking and running are less researched and more controversial than those of pterodactyloids, and we've seen several ideas on this topic emerge since the 1980s. These include non-pterodactyloids as bird-like bipeds (briefly discussed here); as slow and terrestrially ineffective lizard-like quadrupeds; as quadrupeds which were adept climbers; or quadrupeds with sprawling forelimbs but erect hindlimbs, which had to run bipedally. From these, the notion of early pterosaurs as particularly laboured terrestrial locomotors has emerged dominant. In several cases, this perception has been important when interpreting pterosaur history and biology: how early pterosaurs foraged, their roles in Mesozoic ecosystems, and patterns in their fossil record have all factored in the terrestrial ineptitude of the first flying reptiles (e.g. Unwin 2005; Ősi 2011; Butler et al. 2013). There are three core assumptions forming the foundation of non-pterodactyloid terrestrial incompetency:

The absence of pterosaur tracks before the Middle Jurassic demonstrates that these animals rarely walked, allegedly because they were so awful at it

The sprawled limbs of pterosaurs made quadrupedal terrestrial locomotion slow and difficult.

Pterodactyloids, with their fully erect limbs and split uropatagia, freed themselves from these constraints and 'terrestrialised' the group, leading to the formation of a track record which roughly corresponds to the earliest occurrences of pterodactyloid remains.

All this sounds fine, until the realisation hits that many of these assumptions are on somewhat shaky logical ground. Indeed, available data and well-known specimens can be used to undermine them to the extent that we might want to reconsider the whole 'terrestrially inept non-pterodactyloid' concept. In a new paper, published this week in PeerJ, I've attempted to do just that, outlining how we probably know a lot less about the terrestrial prospects of non-pterodactyloids than some suggest, and that some of our chief assumptions about these animals may very well be incorrect. Being a PeerJ paper means you can read the whole thing for yourself, for free (along with the review history) but let's summarise the main points here, tackling each of the points raised above in turn.

1. Large hindlimb membranes ‘shackled’ early pterosaur legs

Pterosaur and bat uropatagia compared. A, tracing of Sordes pilosus fossil, showing one of our best known uropatagia specimens; B, reconstruction of Rhamphorhynchus skeleton, mapped out with likely membrane distributions for non-pterodactyloids; C, terrestrially-competent vampire species Desmodus rotundus, complete with uropatagium rather like that of non-pterodactyloids. From Witton (2015).

Notions that the proportionally large hindlimb membrane of early pterosaurs (above) would impede terrestrial locomotion are not based on much in the way of detailed analysis of pterosaur soft-tissues. It really is just the size of the membrane, and the fact it was anchored extensively across the leg and fifth toes, which suggests it impeded terrestrial activity. Evidence that this membrane was particularly stiff or unyielding has not been presented, unlike other parts of pterosaur wings, which seem to have been stiffened and reinforced by long fibres. To the contrary, several workers have suggested that pterosaur uropatagia were probably as elastic as other membrane tissues close to the pterosaur body (Unwin and Bakhurina 1994) – those which were flexible enough to permit huge strides and running behaviours recorded in pterosaur trackways. In light of this, suggestions that this organ 'shackled' their hindlimbs seem a bit odd.

Moreover, there are plenty of modern animals with analogous uropatagial structures which locomote terrestrially without problem – examples include several bats with terrestrial capabilities described as being ‘rodent-like’. Many of them even make habits of grounded foraging, digging, crevice-crawling, running, climbing and other complex behaviours. All this occurs without their membranes being damaged, snagged or being otherwise restricting locomotion. Much of this is aided, it seems, by uropatagia being elastic, shrinking away when their limbs are not in flight configuration. Of course, there are some animals with large hindlimb membranes which aren’t particularly hot on the ground, but zoologists have labelled aspects of limb strength and myology as more important here than membrane size. This seems to be a second complication for the idea that large uropatagia were problematic for walking in the way suggested by some pterosaur workers. In concert with what we know of pterosaur membrane anatomy, I'm left wondering what, if any, deleterious effect non-pterodactyloid hindlimb membranes had on their terrestrial prospects.

2. Trackways = terrestrial proficiency

I must admit some surprise that a number of pterosaur workers consider trackway presence and abundance to correlate with terrestrial competency. This is especially so because it is widely acknowledged that the pterosaur fossil record, and that of early pterosaurs in particular, is generally poor. This is not to say that a deficit of early pterosaur tracks is definitely meaningless, but negative evidence is only significant when all other factors are accounted for. To appreciate the deficit of early pterosaur tracks as significant, we’d ideally want to have a good understanding of pterosaur ecology and behaviour, as well as their abundances in habitats suitable to track making. Neither of these are the case at present. Moreover, the track records of many Mesozoic animals are poor, even those which, on paper, have vastly greater track-making potential than early pterosaurs. Examples include widely distributed, entirely terrestrial clades like certain dinosaurs, crocodylomorphs, and mammaliaforms. Even relatively recent, large bodied terrestrial animals – like ceratopsids and tyrannosaurs – have track occurrences countable on the digits of one hand, despite their increased bulk ensuring that they leave deeper, long-lasting and theoretically more-preservable prints. Needless to say, applying 'pterosaur logic' to these animals - assuming that they avoided making tracks because of terrestrial ineptitude, and were reliant on other means of transportation - leads to some... interesting ideas on other potential forms of locomotion.

Revised locomotion in tyrant dinosaurs, ceratopsids and mammaliaforms, brought to you by the 'no footprints = terrestrial competency' hypothesis.

Of course, no-one thinks the lack of footprints in these groups is anything to do with terrestrial competency. The fossil record is full of strange quirks reflecting a secret recipe of ancient animal behaviours, taphonomy, sampling, interpretation, and plain serendipity, so scare track data may have no significance at all. Or it might. We don't know, because we can't account for all variables. But that doesn't matter, because what we can do is use available data - that of limb functionality - to draw conclusions about the terrestrial prospects of these groups. That has to trump an absent track record, because we cannot test the significance of our conclusions on a negative dataset. Until we know more about early pterosaur functionality, ascribing the absence of a footprint record to their terrestrial capacity is probably getting ahead of ourselves, and puts the approach of pterosaur researchers at odds with other branches of vertebrate palaeontology.

3.All non-pterodactyloids had sprawling forelimbs

I’m going to cut to the chase here by pointing out that evidence for sprawling pterosaur hindlimbs is not especially strong – most arguments made to support it are now a little old-school, to the extent that they conflict with modern approaches to assessing limb joint function. There’s been a lot of work done on pterosaur hindlimb posture and it seems all flying reptiles had at least upright hindquarters - let's leave that there. But what about sprawling forelimbs? Certainly some early pterosaurs – specifically rhamphorhynchines, and probably ‘campylognathoidids’ - had to sprawl, because their glenoids (shoulder joints) restricted movement of the humerus below the shoulder itself. Their articular surface allows for plenty of room for movement above the shoulder, and some room for fore-and-aft motion, but a large bony ridge prohibits the forelimb adopting anything like an upright pose. This isn't controversial: several authors have already noticed this.

However, several non-pterodactyloid specimens seem to have rather different glenoid morphologies to those of rhamprhorhynchines and 'campylognathoidids'. Species like Dimorphodon macronyx, and perhaps some wukongopterids, have shoulder girdles which lack that ventrally-restrictive bony stop. Instead, their glenoids which are ventrally open, the articular surfaces wrapping around the underside of the glenoid body to meet the shoulder girdle itself rather than a jut of bone. This morphology is borne out by several three-dimensional Dimorphodon glenoid specimens and is unlikely to represent chance distortion of a glenoid into a more 'open' morphology. These specimens seem to indicate that at least some early pterosaurs were capable of tucking their forelimbs underneath their bodies.

Variation in early pterosaur shoulder girdles. Images on the left shown the shoulder joint of Dimorphodon macronyx - note how the (shaded) articular surface extends to the underside of the glenoid (bottom image, and left photograph) to make a ventrally-open shoulder joint. By contrast, the glenoids of certain other non-pterodactyloids (images top right) have articular surfaces only on the dorsal and lateral glenoid surface: the ventral is blocked by a bony ridge. In this morph, the humerus clearly cannot rotate much beneath the shoulder at all. A schematic of the ranges of motion offered by these two shoulder types is shown at bottom right. From Witton (2015).

It’s not a given, however, that the ability to fully adduct a limb correlates with a habitually upright stance: range of motion alone tells us little about habitual joint postures. Is there anything in early pterosaur anatomy to suggest which forelimb postures these animals preferred? Potentially, yes: the end of their humeri. Recent work on the forelimbs of quadrupedal tetrapods has identified anatomical correlates of routine sprawling and erect postures in elbow skeletons (Fujiwara and Hutchinson 2012). Because these joints have to take the strain of standing, different poses emphasise the development of characteristic muscle groups and their corresponding bony attachment sites. Having positively tested this thoroughly on modern animals, we can start to use it on fossil ones, including pterosaurs. Fujiwara and Hutchinson (2012) have already looked at pterodactyloid humeri and suggest they have all the right features for an upright limb. Ergo, if we see pterodactyloid-like humeral morphologies in earlier pterosaurs, they might have had upright limbs, too.

As with shoulder anatomy, it turns out there some potentially significant variation here. Rhamphorhynchines and (probably) 'campylognathoidids' - taxa seemingly confined to sprawling - have humeri which are quite different from those of pterodactyloids. Their elbows are very narrow, being little wider than the condyles necessary to articulate the forearm. This fits well with the predictions of Fujiwara and Hutchinson, because we would expect pterosaurs confined to sprawling to have different anatomy to tall-standing pterodactyloids. Dimorphodon and wukongopterids - those pterosaurs with less restrictive shoulder joints - have a different elbow anatomy however: a distal humerus which is broadly expanded beyond the margins of the forearm articulations. In virtually all details, these humeri have distal ends very similar to what we see in pterodactyloids and, all else being equal, that might imply a similar loading regime at their elbows. We might take this an an indication that upright postures were adopted regularly in these non-pterodactyloids, which marries nicely with observations made about their ability to tuck their forelimbs under their bodies. The idea that Dimorphodon and wukongopterids were also sprawlers is less parsimonious because we have to explain why some early pterosaur humeri don’t resemble those of obligate sprawlers, but instead look so similar to those of pterosaurs we’re confident had erect forelimbs.

This observation might be quite significant for considerations of terrestrial abilities in early pterosaurs. Sprawling postures are not as restrictive to locomotion as some pterosaur literature suggests (sprawling does not limit its users to slow crawling, and is actually quite useful for certain habits, like climbing or accelerating quickly), but there might be something to the idea that upright locomotion is better for sustained, active terrestrial habits. Indications that the likes of Dimorphodon and wukongopterids were capable of walking on fully erect limbs, without impedance from their uropatagia, suggest they have greater terrestrial potential than previously anticipated. Perhaps our views on the likely habits of non-pterodactyloids might benefit from further research.

Further indications of terrestriality?

The possibility that some early pterosaurs had erect forelimbs is only one indication that these taxa may have been more terrestrially adept than we’ve previously considered. I think our views on these animals have been biased by the familiarity of taxa like Rhamphorhynchus: these sprawling, slender-limbed species with oversize forelimbs don’t look like the hottest terrestrial locomotors in town (see Dorygnathus illustration, below), and this perception may have bled into our consideration of non-pterodactyloids as a whole. In actuality, taxa like Dimorphodon, Preondactylus, anurognathids and others have pretty chunky, proportionate limbs: we could take their wing fingers away and they still look capable of looking after themselves using terrestrial locomotion alone. Several authors have noted features indicative of strong running and leaping abilities in early pterosaur hindlimbs, and this might apply to their forelimbs too.

It’s also intriguing to note that some features found in pterosaur digits – sesamoids immediately above and behind their claws – are only otherwise found in terrestrial reptiles (lots of squamates and one fossil stem-turtle, Proganochelys). No-one seems to know exactly what function these antungual sesamoids perform, but I've speculated that they are related to claw contact with hard surfaces. Sesamoids seem to mostly offer two functions: enhancing joint leverage or protecting tendons from extremes of motion. Claws routinely deflected backwards by contact with substrata might necessitate a bony element in the tendon to ensure nutrient flow to the tendon is maintained, or else to facilitate better leverage when actively hyperextending claws to prevent claw blunting. It's difficult to think what else a sesamoid above and behind a claw can really help with, especially given their occurrence in animals as different as pterosaurs, lizards and a stem-turtle. I'm not the first to suggest pterosaurs could retract their claws (Frey et al. 2003), and some sort of hyperextension would explain the large digital articular surfaces seen in pterosaurs with antungual sesamoids compared to those lacking them. Again, there is clearly need for more research here, but the take-home is that this anatomical aspect of non-pterodactyloids is only mirrored in terrestrial animals, and might present another feature signifying greater terrestriality than expected in early pterosaurs.

Tying this all together

A 'traditional' view of a non-pterodactyloid standing posture: Dorygnathus banthensis with sprawling forelimbs. How the development of this feature fits into pterosaur evolution remains to be seen: earlier pterosaurs (both stratigraphically and phylogenetically) may have stood quite differently. Prints of this image are available.

It's noteworthy that features proposed as potentially signifying terrestriality in early pterosaurs are not neatly mappable to any current concept of non-pterodactyloid phylogeny. Even comparatively simple models of early pterosaur evolution show complex changes in shoulder girdle morphs, humeral anatomy and and limb robustness as we trace evolutionary pathways up the tree. Nevertheless, the potential for upright postures, running behaviours and other features of proficient terrestriality seem deeply rooted in Pterosauria because some of the oldest, 'most basal' pterosaurs possess such indicative anatomy.
A lot of what is said here complements ideas already in pterosaur literature. To use a well-studied example, there are lots of hints that the Lower Jurassic Dimorphodon was terrestrially adapted: it seems proportionally heavy to the extent of impeding flight potential, has been strongly suggested to have a diet of insects and small vertebrates, and has a suite of features suited to climbing (see the previous post). Combine these with the possibility of erect limbs, subcursorial limb proportions, robust extremities and so on, and we've got a pretty good argument for Dimorphodon being terrestrially competent, and maybe even adapted for a primarily terrestrial existence. It's hard to think of a lifestyle more opposing to traditional interpretations of non-pterodactyloid palaeobiology than that, but a multitude of disparate research projects seem to be collectively pointing that way.

In all, my point for writing this synthesis paper is to demonstrate how little we've really looked into the terrestrial prospects of early pterosaur species. If even basic variation in their limb arthrology remain poorly studied, how can we claim to understand their terrestrial prospects or plug our models into our Big Picture of Pterosaur Evolution? Even if everything I've said here ends up being challenged, I hope at least this new paper stimulates some detailed research into this rather poorly explored area of pterosaurology. Until that happens, my suggestion is that we avoid blanket-statements about the terrestrial prospects of non-pterodactyloids.

Those interested in early pterosaur funky morph might be interested to know that I'm talking about the flight performance of Dimorphodon at Flugsaurier 2015. Registration is open until 30th June, so get your interest registered quickly!

Monday, 8 June 2015

How to make Dimorphodon macronyx fly: chase it down with a Sarcosaurus-like dinosaur. The most recent illustration of the 'reluctant flier Dimorphodon' hypothesis, based on predicted wing parameters of this heavyset pterosaur. Prints of this image are available.

With Jurassic World about to start assaulting the box office and intelligence of palaeontologists around the globe, it seems appropriate to take a look at some of the science behind the animals featured in the film. Being just about to move house (copious books and fossils = Worst. Moving. Experience. Ever.) means I can't write about them all, but we have time to look at one of the pterosaurs they're featuring, and coincidentally also one of my favourite fossil species: Dimorphodon macronyx. I was quite chuffed to hear Dimorphodon was going to make it to the big screen, but... oh dear. Poor Dimorphodon has been really mangled by the infamous reconstruction approach of the Jurassic World film makers, and the information on their website is really awful - powerful talons for snatching fish? Seriously?. From what we've seen so far at least, I wonder if it's one of the worst reconstructions in the film.

Clearly, the Dimorphodon of Jurassic World is going to be nothing like the Dimorphodon known to researchers. OK, that's hardly a shock, but it's a shame nonetheless. Dimorphodon is not a theropod-headed scaly dragon, but an especially interesting and significant animal to pterosaur researchers. I'm involved in several Dimorphodon related projects at the moment - one should see fruition next week - and thought I'd share some of the basis for my fascination here.

OK, smart guy, what was Dimorphodon really like?

Dimorphodon is one of the best known early pterosaurs. Seemingly unique to Lower Jurassic rocks of Dorset, UK (Mexican material previously referred to Dimorphodon likely represents a different taxon), it is perhaps the oldest pterosaur known from anything like three-dimensional remains. This doesn't include skull material, which is always preserved with the topography of a pancake, but much of our Dimorphodon limb and body fossils have some, if not entire, three-dimensionality to them. Although a complete skeleton has never been found, several half- or near-complete specimens are known along with a lot of associated material. The upshot of this is that a fairly decent understanding of Dimorphodon osteology has been held for almost 150 years (so, yeah, the Jurassic World animal is less accurate than renditions put together by Victorian palaeontologists. It's not the only Jurassic World species to suffer this sort of problem). With most older pterosaur fossils being either mere fragments or entirely squashed skeletons, Dimorphodon represents an important insight into early pterosaur anatomy. This is especially so because some aspects of its skeleton - particularly jaw shape, dental anatomy and wing proportions -indicate it is a rather 'plesiomorphic' species, closely related to some of the oldest known pterosaurs, such as the Triassic taxa Peteinosaurus and Preondactylus. This might make it particularly informative as goes the anatomy of the first pterosaurs, with all sorts of potential for investigating their locomotion and ancestry. It must be said that this is only one interpretation of Dimorphodon phylogenetics however: the interrelationships of early pterosaurs are particularly contentious, and other workers suggest it plots much further away from the base of the pterosaur tree.

Restored Dimorphodon macronyx skeleton. From Witton (2013).

Dimorphodon is widely known for its dentition, its 'two form teeth' providing a generic namesake. The larger teeth of Dimorphodon are sometimes incorrectly portrayed as splaying from its jaws, somewhat like those of rhamphorhynchine or ornithocheird pterosaurs. So far as we can tell, though, they were more-or-less vertically orientated. These bigger teeth possess carinae - cutting surfaces running along the anterior and posterior dental margins. Only the posterior region of the lower jaw has the second type of tooth - very small, sharp cusps which are positioned at regular intervals to make the jaw resemble a hacksaw blade. These were clearly the subject of heavy use in life: some specimens possess broken tips.

D. macronyx tooth morphologies. Note the broken tooth exploded from the main image. From Ősi (2011).

Its not only teeth which make Dimorphodon characteristic, however. The size of the skull is quite remarkable compared to other early flying reptiles, and a forerunner to the trend of large skull sizes that would develop later in monofenestratan pterosaurs. The fact all Dimorphodon skulls end up being flattened indicates that the skull bones were not robust. That said, although likely full of air in life, the skull of Dimorphodon is still large enough to occupy a proportionally large amount of body mass, as were the hindlimbs (Henderson 2010). We tend to think of early pterosaurs as scrawny-legged animals which couldn't walk if their lives depended on it, but Dimorphodon limbs are pretty well built. Indeed, the hindlimbs are so strongly put together that Dimorphodon was the pterosaur behind the controversial 'dinosaur-like bipdeal pterosaurs' concept discussed through the 1980s and 1990s. I've been wondering of late whether we can consider that idea fully refuted now: at least one individual still champions the idea, but they have not really countered the wealth of evidence set against pterosaurian bipedality. For those not keeping score, that evidence includes the anterior centre of gravity occurring in all pterosaurs; issues with hindlimb musculature efficiency at poses imposed by bipdal gaits; problems with neatly folding the wing; the wealth of quadrupedal pterosaur trackways; trackway and osteological characteristics indicating plantigrade feet; and scaling regimes of pterosaur limbs matching those of quadrupedal volant animals, but not bipedal ones. Although a minority of these points have been partially refuted (sometimes controversially), evidence supporting the bipedal pterosaur hypothesis is thin on the ground compared to that for quadrupedality, and this applies to Dimorphodon as much as anything. I'm coming to the opinion that the use of bipedal or quadrupedal gaits is not really a debated topic for pterosaurs now.

The hands and feet of Dimorphodon are also robust, and equipped with large, trenchant claw bones (these, of course, provide the specific namesake, 'macronyx'). There are indications that the extensor muscles controlling these might have been powerful, as every claw on both hands and feet is equipped with a neighbouring sesamoid - those intra-tendinous bones serving to enhance muscle output or protect tendons against powerful joint motion. Interestingly, the only other animals with these claw-adjacent sesamoids are lizards and a 'bottom walking' fossil stem-turtle - more on that another time. As with all pterosaurs, there is no indication that their hands or feet were for grasping, and their claws are really nothing like talons (take that, Jurassic World website).Dimorphodon wings are interesting for their contrasting proportions to the rest of the body, as well as those of most other pterosaurs. Although the wing fingers of Dimorphodon are decently sized - they occupy over half the length of the entire arm - the overall wing length is a bit on the small size, at least compared to predicted Dimorphodon masses. At least 3 studies have independently predicted relatively high wing loading in Dimorphodon, suggesting those relatively big skulls and legs were not accommodated for with increases in wingspan. First-principle interpretations of these results - that Dimorphodon flight may have been a bit more fraught and energy-demanding than similarly-sized pterosaurs - is being borne out in assessments of wing shape (Witton 2008) and flight studies (which I'll be talking about at Flugsaurier 2015). I went so far in 2008 as to suggest that Dimorphodon was a 'reluctant flier', because its predicted wing parameters seemed to closely match those of game birds and other woodland avians - those which take flight when they have no alternative, and keep their flight durations short (see illustration at top of post). Early indications from more detailed assessments are that aspects of flight we normally assume for pterosaurs - soaring and gliding - may well have been challenging, or effectively impossible, for Dimorphodon. These predictions of a heavyset pterosaur by myself and others are something of a first for flying reptile studies: mostly, we've remarked about how lightweight and glide-efficient pterosaurs were, not the opposite.

The Puffinodon: another palaeoart meme?

What sort of lifestyle did Dimorphodon lead? Considering we're talking about a pterosaur, you can almost guarantee that someone has proposed that Dimorphodon ate fish. Some authors - perhaps Bakker (1986) was the first - have noted similarity between the skull of Dimorphodon and that of puffins, taking this to mean that these animals lived similar lives of diving into the water in pursuit of nektonic prey. Lots of artists have been inspired by this idea.

The palaeoart meme of 'Puffinodon'. Note that related puffin-inspired Dimorphodon art, not shown here, exists where dark body colours contrast with a variably coloured, vertically-striped bill. I'm as guilty of the latter as anyone.

The 'Puffinodon' concept doesn't do very well under testing. For one, the skulls of Dimorphodon and puffins aren't really that alike. Most of what makes up the deep cranial profile of puffin bills is soft-tissue, not bone. Moreover, puffins and other diving birds have wings well-adapted for 'flight' under water in that their wing bones are somewhat flattened, with thick bone walls. Dimorphodon wings, by contrast, are actually broader in some respects than those of other pterosaurs (stay tuned for more on that), and there are no indications that it had thickened bone walls. To the contrary, there are indications that its postcrania was pneumatised, at least in part.

So what did Dimorphodon eat? A comprehensive study of early pterosaur skulls and teeth concluded that Dimorphodon jaws were well-suited to a diet of insects, carrion and small vertebrates (Ősi 2011). That actually chimes pretty well other interpretations of Dimorphodon palaeobiology: there are indications that Dimorphodon was an adept climber, a fast runner, and - as discussed above - possibly flight restricted. A diet of insects and small vertebrates fits with these assessments of locomotor habits suited to terrestrial realms pretty well, and we might imagine Dimorphodon as better adapted to chasing down lepidosaurs and large beetles than it was diving for fish. Indeed, there are some pretty cool aspects of Dimorphodon anatomy indicating it may have been really at home on land - as in, as much as pterodactyloids were. This might come as a surprise to some, seeing as non-pterodactyloids have largely been thought of as terrestrially inept. We'll have to wait just a bit longer before I can talk about those, however.

So that's Dimorphodon in a few paragraphs, then: nothing like the animal we'll be seeing this summer at the cinema, and perhaps nothing much like other pterosaurs, either. If early pterosaurs and their lifestyles are your thing, stay tuned for some new ideas on that very soon.

Wednesday, 3 June 2015

Last week we took a look at some new art of animals from the Wealden Supergroup, the intensively studied, historically important Lower Cretaceous rocks of Southern Britain. We all know the Wealden for celebrity dinosaurs like Iguanodon and Baryonyx, but there's a heap of other interesting animals in there which get relatively little publicity. It's mostly these we're focusing on here, in the second (and final) part of these 'picture of the day'-style posts.

As before, if you like anything here, remember that you can buy prints of them all from my shop (the Wealden section might be relevant) and its new Facebook outlet. Indeed, if you like my work and are on Facebook, why not 'like' the new Mark Witton Palaeoart page? It's the best place to see when new prints and finished pictures are available.

Baryonyx walkeri: king of the fishers, redux

Baryonyx walkeri, off for a stroll among the crocodyliforms and pterosaurs.

Let's break this post in with a familiar animal: spinosaurid Baryonyx. It's hard to appreciate now how weird this animal seemed back in the 1980s and 1990s. At this point, other spinosaur material was only very poorly known, and laymen and scientists alike found this weird, superficially-crocodile like animal fascinating. Ironically, it's recently turned out that we first collected Wealden spinosaur material centuries ago, but struggled to recognise its significance until more complete remains were unearthed in the 1980s. We now know that Baryonyx can be found throughout a good chunk of upper Wealden stratigraphy and teeth referable to it - or another spinosaurid - are fairly common, at least as Wealden dinosaur fossils go. Baryonyx provided the basic template we'd recognise for all spinosaurid anatomy until last year when, famously, some spinosaurs were proposedto be rather different. It's clear that, whatever is going on with Spinosaurus, Baryonyx retains more conventional hindlimb and pelvic proportions, and may not have been so aquatically adapted as true spinosaurines. In this updated image, B. walkeri is splashing into a body of water while goniopholidid crocodyliforms and gnathosaurine pterosaurs go about their business around it. Note how much larger Baryonyx is compared to the crocs: Baryonyx is the largest theropod in the Wealden Supergroup, by a good margin.

Last year I was lucky enough to provide the first restoration of Kompiodontosuchus aprosdokiti, a small neosuchian crocodyliform common to the Wessex Formation, and perhaps other parts of the Wealden sequence. Koumpiodontosuchus is a bernissartid, a group of small-bodied crocodyliforms with robust, shell-cracking teeth at the back of their jaws. As you'll know if you read my write up last year, these were likely employed in smashing molluscs and insects. The tetanuran theropods in this image are unnamed, but are not thought to be referable to any existing Wealden taxa. We probably need more material of them to consider them nameable, however: recognising that they are different from other Wealden theropods is only half the battle. Modern students of Wealden fossils famously do their best to preserve historic names based on fragmentary bones, but there seems to be an effort to 'future proof' Wealden taxonomy against confusion by only naming well-represented, characteristic animals. I guess I could have chosen one of the better known theropods to play the 'This was the Age of Dinosaurs' card for this PR image, but I think it's good to show that not all large theropods in the Wessex palaeobiota were Neovenator, Baryonyx or Eotyrannus.

Welcoming the new Wealden ornithomimosaurs

A flock of Wessex Formation ornithomimosaurs forage in a marshland, while istiodactylid pterosaurs skulk about behind them.

Those keeping their ears to the ground will know that the newest arrivals to the Wealden dinosaur palaeobiota are ornithomimosaurs, commonly known as ostrich dinosaurs. Two specimens show that these animals were present in both the Weald and Wessex basins of the broader Wealden succession, and one of these fossils represents a historic taxon named in 1889: Valdoraptor oweni. Key to identifying ostrich dinosaurs in the Wealden was the discovery of abundant ornithomimosaur remains in France, many of which are so reminiscent of Valdoraptor and other Wealden theropod material that they may represent the same taxon. If you want to know more about these and their relationship to the complex story of Wealden theropods, check out Darren Naish's post on this at Tetrapod Zoology.

The above new painting shows a group of (nameless) Wessex Formation ornithomimosaurs in a well-vegetated marshland, in the rainy season, while istiodactylid pterosaurs mosey about in the background. The abundance of ostrich dinosaurs and juveniles in the middle-right are nods to the frequent recovery of abundant specimens of different levels of maturity at many ostrich dinosaur sites, including the new, French 'Angeac ornithomimosaur'. Note that the wings of the running foreround animal are somewhat swept back: I don't think the more common way of reconstructing ornithomimosaurs with 'dangly arms' looks right. They look like they should be holding shopping bags or something.

Valdosaurus in the forest, redux

Two Wealden dryosaurids Valdosaurus canaliculatus, and a stubborn avialan.

Ornithomimosaurs weren't the only fast runners in Wealden landscapes. Dryosaurids, like Valdosaurus canaliculatuswere also fleet-footed animals with powerful, well-muscled hindlimbs, and tiny bodies attached to the front. In this reworked image, two of these 3-4 m long animals are taking it slow through a Wealden woodland. Although Wealden climates were quite warm and arid, leaving much of the landscape looking quite chaparral-like, some relatively upland parts seem to have been more vegetated: it's here that this picture is set. In my mind, these animals always walked with the stooping posture of the foreground animal - as noted last time, I like the idea that prehistoric animals had characteristic postures varying slightly from those we consistently restore in skeletal restorations. Note the avialan on the left of the image, which is a nod to the recovery of bird teeth from Wealden deposits. Anyone who's ever been forced to walk around a stubborn reclined mallard will recognise the situation now facing the Valdosaurus.

Barilium dawsomi in leathers, redux

Barilium dawsoni, a large and very robust iguanodont from Sussex. A flock of 'Ashdown maniraptorans' add scale.

Last time we featured Iguanodon bernissartensis: now it's the turn of the 'other' big Wealden iguanodont, the stratigraphically older, and osteologically chunkier Barilium dawsoni. In this redone painting, I've tried to make the Barilium skin more interesting than just plain old scales, covering the back in small, horny ossicles and creasing the flanks as if the skin is particularly thick, leathery and folded. I think we should be rendering more interesting skin regularly in scaly dinosaur palaeoart, as it seems most extensive dinosaur skin remains show unexpected features - strangle scales, wattles, folds and that sort of thing - which small skin patches mostly cannot record adequately. It's interesting to contrast these skin impressions with homogeneous restorations of scaly dinosaur appearance presented by some, where every species is covered in smooth hide following perfect contours of the underlying tissues: I'm not sure that's what fossils are telling us. As before, the 'Ashdown maniraptoran' provides scale to the bulk of Barilium. For the uninitiated, the Ashdown maniraptoran is seriously small for a Mesozoic dinosaur - maybe about 30-50 cm long. If you find big iguanodonts exciting, be sure to check out this previous post.

Polacanthus redux, again

A Wealden tree vies for attention with Polacanthus foxii, and some tiny birds.

OK, I'm cheating a bit with this one. This redone version of a much older painting has been posted fairly recently, but it seemed a bit remiss to skip this ankylosaur in this run down of recently produced Wealden palaeoart. Polacanthus foxii is, of course, the Wealden's sacral-shield-bearing nodosaurid, shown here strolling around a Cretaceous hillock with some birds for company. Having scratched the completist itch, let's move on, because we've seen this all before.

Accidentally sinister Leptocleidus, redux

Our final stop is in Wealden rivers and estuaries, where Leptocleidus superstes and other species of freshwater leptocleidid plesiosaurs roamed. The new version of this image has added a lot of detail on top of the original, which has inadvertently made the mother and calf Leptocleidus look more sinister than intended - hey, it's not my fault their teeth stick out like that. Back in the original post on these animals I mentioned that pliosaurs may also have been present in Wealden lakes and rivers, but note that this is no longer certain: the Hastanectes valdensis remains once provisionally considered pliosauroid have been placed in Leptocleididae in more recent analyses. That does make for a neater story - it means that leptocleidids retain their dominant role as 'near-shore-or-freshwater' animals, but perhaps a slightly less interesting one.

And that's all for now - I hope you've enjoyed this jaunt back to the ancient Wealden and these revised artworks. I'm sure we'll visit the Wealden again in time. Coming next, probably: walking with non-pterodactyloid pterosaurs.

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Who is this 'Mark Witton' chap?

Dr Mark Witton is a palaeontologist and palaeoartist, affiliated with the University of Portsmouth, UK. My technical research is focused on pterosaurs - Mesozoic flying reptiles - but my artwork has introduced me to a wide array of different fossil animals that are just as interesting. I work as a freelance author, consultant and artist: check out my work at MarkWitton.com, follow me on Twitter @MarkWitton, and browse my books here. Contact me at wittonprints[at]gmail.com. Due to volume of email I can't always reply to messages, but I do my best.